Must bevel be used in every situation?
Yes. No. It depends.

Objects with bevel are indeed more realistically looking and as probably you know by now, there is not a single known object perfectly sharp. Even sharpest knife in some tremendous magnification will show some "bevel" on its edge.

Question is, when do you need to show it. Firstly lets get trough most commonly used bevel cases.

Bevel High Poly



Most natural one from the bunch. I've used on this example 3 segmented Bevel and 2nd level of Subdivision Surface modifier. It goes to whopping 3,072 tris on a simple cube. 

It looks the best out of every other case but it is heavy as hell. 


  When to use it? If you are rendering extremely realistic object in a
  closeup or try to keep realism overall at high level.


Bevel Baked



This one is/was mostly used in game industry. It gives you considerably good looking bevel from the distance. This object has only 12 tris! Problem is - you will loose object silhouette and it is kinda not user friendly in Blender.


  When to use it? In game engines for non significant distant objects.


How to: How does normal baking work?

Bevel Shader



This is one of the newest additions to Cycles. It's very similar to the baked one but you have nice control over it. You don't need to create normal maps images, high poly meshes etc. Cons - you can't export it that way, so baking still will be needed for external software and still you will lose object silhouette 


  When to use it? Non significant distant objects in Cycles rendering.
  EEVEE doesn't support it (yet?).


How to: https://docs.blender.org/manual/en/latest/render/cycles/nodes/types/input/bevel.html

Bevel Weighted (Hardened Normals)



Also one of the newest things in Blender and by far my favourite. You can achieve it by Bevel modifier in 2.80 (Hardened Normals) or with Weighted Normal Modifier.

This option gives you light mesh (44 tris with 1 segment bevel, 108 tris with 2 segments - example is 2), silhouette and smooth looking edges. 

It was highly used in a game Star Citizen, you can read more about it here.

Overall it's not a perfect solution. I don't know how compatible it is with main game engines besides UE4 (which support it) but I know for sure LuxCoreRender can't handle Custom Normals Data.


  When to use it? Games, non significant objects in middle range (with 1
  segment), close non significant objects with more then 1 segment.


Summary

As you can see there is no single answer to your question. You need to choose accordingly to your needs.

I've presented only selected methods easily achievable in Blender. There are more of course.

Or maybe you don't need bevel at all for small details in background - it's your call.

Side note: There are of course more pros/cons and also use cases for every method. I've pointed most relevant from my point of view. If you think that something else should be written in this answer, let me know in comments section.